South Australian researchers are tackling the question of how much nitrogen is available to wheat crops, to give growers a benchmarking tool.
Associate Professor Victor Sadras
The South Australian Research and Development Institute’s Dr Victor Sadras, who heads up the GRDC-funded project team, says benchmarking is important to identify nitrogen status.
While nitrogen fertiliser is commonly used to compensate for both the characteristic low fertility of many Australian soils and the extraction of nitrogen by grain, the use of nitrogen can be constrained by uncertain rainfall. So, it is not uncommon for wheat crops to display nitrogen deficiency in seasons with average or above-average rainfall.
This benchmarking research builds on a previous GRDC project (DAS00089), which showed that new wheat varieties have an increased capacity for uptake of nitrogen from soil. National Variety Trials (NVT) observations in low-nitrogen soils have also highlighted reductions in grain protein of modern varieties such as Mace and LongReach Scout.
Understanding the nitrogen requirements for different varieties is important for adjusting inputs to meet the needs of the variety sown.
“Despite the importance of nitrogen for crop yield and its incidence in the variable costs of farm business, there is no established benchmark to assess the nitrogen status of crops,” Dr Sadras explains.
Although fertiliser guidelines have improved over recent years, with emerging technologies based on canopy size and greenness, yield–nitrogen relationships remain the main reference.
“A limitation of this approach is that yield–nitrogen relations are highly variable with soil, crop and season, and do not fully account for crop physiology,” Dr Sadras says.
He says physiological principles show a tight loop between crop growth, which stimulates nitrogen uptake, and nitrogen uptake, which favours crop growth. For this reason, the nitrogen status of the crop must account for crop biomass. Comparing nitrogen status where factors other than nitrogen (for example, sowing date and variety) affect growth is misleading unless crop biomass is accounted for.
Dr Sadras says the direction of the plant breeding industry – breeding for yield – is probably shifting the biomass–nitrogen balance in crops, so agronomic practices, including fertilising, need to be adjusted to capture the potential yield and protein content of new germplasm.
“The underlying, most basic question is, what is the current nitrogen status of wheat crops in growers’ fields and in the NVT?”
The project aims to fill this gap through a two-step approach. First, by developing nitrogen dilution curves that relate crop nitrogen concentration and biomass (Figure 1), and then benchmarking crops against these curves to derive a nitrogen nutrition index.
“The nitrogen nutrition index is the only method to unequivocally measure the nitrogen status of the crop, and removes much of the uncertainty related to soil and seasonal conditions that are typical of conventional crop diagnostics,” Dr Sadras says. “This method could help improve diagnostic criteria and management of nitrogen fertilisation in both growers’ fields and the NVT.”
Trials are being carried out at Hart Field Site, NVT sites and growers’ paddocks in SA. The research team comprises Dr Sadras, Marianne Hoogmoed, Rob Wheeler, Lachlan Lake, Sarah Noack, Peter Hooper and Barry Mudge.
Figure 1 Crop nitrogen concentration and biomass
The concentration of nitrogen in shoots required to achieve maximum growth declines with increasing biomass (black line), because biomass grows faster than nitrogen uptake and young crops are mostly leaves, with high nitrogen concentration. After stem elongation, the leaf:stem proportion declines, and the lower nitrogen concentration in stems drives the whole-shoot nitrogen concentration down.
As the crop grows bigger, there is relocation of nitrogen from shaded leaves in lower position of the canopy to well-illuminated leaves at the top of the canopy. Points below the curve are nitrogen deficient, and points over the curve have excess nitrogen.
The red point represents a nitrogen deficient crop with biomass B, and actual nitrogen concentration (Na). To achieve maximum growth, the crops would need to reach the critical nitrogen concentration (Nc). The ratio Na/Nc is a measure of nitrogen deficiency and is called the nitrogen nutrition index (NNI). The NNI removes variation associated with soil and weather, and provides a strong and repeatable measure of crop nitrogen status for benchmarking.
Dr Victor Sadras,
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